The present invention generally relates to polishing a surface of a workpiece. More particularly, the invention relates to improved methods and apparatus for distributing fluids, for example slurry, to the surface of a polishing pad during chemical mechanical polishing.
Chemical mechanical polishing or planarizing a surface of an object may be desirable for several reasons. For example, chemical mechanical polishing is often used in the formation of microelectronic devices to provide a substantially smooth, planar surface suitable for subsequent fabrication processes such as photoresist coating and pattern definition. Chemical mechanical polishing may also be used to form microelectronic features. For example, a conductive feature such as a metal line or a conductive plug may be formed on a surface of a wafer by forming trenches and vias on the wafer surface, depositing conductive material over the wafer surface and into the trenches and vias, and removing the conductive material on the surface of the wafer using chemical mechanical polishing, leaving the vias and trenches filled with the conductive material.
A typical chemical mechanical polishing apparatus suitable for planarizing the semiconductor surface generally includes a wafer carrier configured to support, guide, and apply pressure to a wafer during the polishing process; a polishing compound such as a slurry containing abrasive particles and chemicals to assist removal of material from the surface of the wafer; and a polishing surface such as a polishing pad. In addition, the polishing apparatus may include an integrated wafer cleaning system and/or an automated load and unload station to facilitate automatic processing of the wafers.
A wafer surface is generally polished by moving the surface of the wafer to be polished relative to the polishing surface in the presence of the polishing compound. In particular, the wafer is placed in the carrier such that the surface to be polished is placed in contact with the polishing surface and the polishing surface and the wafer are moved relative to each other while slurry is supplied to the polishing surface.
The distribution of slurry over the polishing surface has been shown to be a critical factor in the chemical mechanical polishing process. The material removal rate across the surface of the wafer is generally related to the amount of slurry received by the polishing surface. Areas on the polishing surface having additional slurry will typically polish the wafer faster than areas on the polishing surface having less slurry. While the material removal rate may be fine tuned by intentionally adjusting the slurry distribution across the polishing surface, it is desirable to have a substantially uniform slurry distribution across the polishing surface.
One approach to distributing slurry across a polishing surface involves depositing the slurry from above in the middle of the polishing surface. Polishing surfaces typically move, for example, in a rotational, orbital or linear motion. The motion, in addition to removing material from the front surface of the wafer, helps to distribute the slurry across the polishing surface. However, this approach leads to a concentration of slurry in the middle of the polishing surface with the concentration of slurry declining in relation to its distance from the middle of the polishing surface.
Another approach to distributing slurry across a polishing surface involves pumping slurry from a cavity below the polishing surface through apertures in a platen and polishing surface to the polishing surface. However, the motions previously mentioned cause the slurry to concentrate along the periphery of the cavity and therefore, when forced to the polishing surface, the slurry is concentrated along the periphery of the polishing surface. As a partial correction for this problem, a cut o-ring has been spirally inserted into the cavity to reduce the concentration of slurry at the periphery of the polishing pad. However, the optimum shape of the cut spiral o-ring is difficult to determine and the optimum shape changes with different slurry delivery rates, speed of motions and types of slurry.
Another problem with using the cavity to distribute the slurry is the time it takes to change from a first slurry reaching the surface of the polishing pad to a second slurry reaching the surface of the polishing pad. Applicant has noticed the delay is caused by the cavity having a volume filled with the first slurry that must be completely replaced by the second slurry. The Applicant has also noticed the problem is compounded by parts of the cavity having no real flow direction resulting in a turbulent fluid motion. The turbulent fluid motion results in a mixing of the slurry and an additional time period when both slurries are delivered to the polishing surface further lengthening the time for a complete slurry change over.
What is needed is a method and apparatus for uniformly delivering a fluid to a polishing surface without being unduly affected by slurry delivery rates, speed of motions or types of slurry. The method and apparatus preferably allow a change in slurry to be quickly accomplished.
The present invention provides improved methods and apparatus for chemical mechanical polishing of a surface of a workpiece that overcome many of the shortcomings of the prior art. While the ways in which the present invention addresses the drawbacks of the now-known techniques for chemical mechanical polishing will be described in greater detail hereinbelow, in general, in accordance with various aspects of the present invention, the invention provides an improved method and apparatus for controlling the distribution of a fluid across a polishing surface.
The invention may be used as a fluid delivery system for delivering a fluid to a top surface of a polishing pad in a chemical mechanical polishing tool. Fluid may be communicated to the top surface of the polishing pad through a plurality of apertures in the polishing pad. The number, size and shape of the apertures in the polishing pad may be varied depending on the desired fluid distribution. The top surface of the polishing pad may also have XY grooves or channels to assist in the distribution and flow of the fluid across the top surface of the polishing pad.
The polishing pad may be supported by a plurality of stacked layers. The stacked layers may be used to support the polishing pad and communicate fluid to the polishing pad. The fluid is communicated through a network of grooves in each of the plurality of stacked layers. The grooves in each layer are positioned and made deep enough so that they may distribute fluid through them to the polishing pad.
In a preferred embodiment, the stacked layers may advantageously comprise one or more subpolishing pads. A subpolishing pad may be used to create two layers by creating one set of grooves on a bottom surface of the subpolishing pad and another set of grooves on a top surface of the subpolishing pad. The grooves are made deep enough in the subpolishing pad to allow fluid to flow from the grooves in the bottom surface of the subpolishing pad to the grooves in the top surface of the subpolishing pad. Each subpolishing pad may also be used to create a single layer by having grooves that are as deep as the subpolishing pad.
A platen may be used to support the plurality of stacked layers and the polishing pad. The platen preferably has a rigid planar surface made of a noncorrosive substance, e.g. titanium, stainless steel or ceramic, for supporting the stacked layers and the polishing pad. The platen may have at least one aperture in fluid communication with the grooves in the plurality of stacked layers. The number, size and location of the apertures in the platen may be varied, but a single aperture below the center of the polishing pad is preferred. However, at least one aperture in the platen must be in fluid communication with at least one groove in the layer closest to the platen.
The size, position and number of apertures in the platen and the polishing pad and the size, position and number of grooves in each of the layers may be varied to control the distribution of fluid across the top surface of the polishing pad. The fluid flows from an aperture in the platen, through the grooves in the various layers, and finally through apertures in the polishing pad to reach the top surface of the polishing pad. In a preferred embodiment, the distance a fluid must travel from the platen aperture through the grooves to any of the apertures in the polishing pad is substantially the same. This embodiment will create a substantially uniform delivery of fluid to the top surface of the polishing pad even when the platen, plurality of layers and polishing pad are moving. This is desirable as polishing pads are commonly orbited, rotated or moved linearly.
A fluid source may be used to store fluid, e.g. deionized water or slurry, to be transported to the top surface of the polishing pad. The fluid source may have a pump for pumping the fluid from the fluid source through a fluid communication path to an aperture in the platen. The fluid source may also have a flow regulator that controls the rate of flow of the fluid through the fluid communication path to the aperture in the platen.
A motion generator may be operably connected to the platen for causing relative motion between the wafer and the top surface of the polishing pad. The motion may be, for example, orbital, rotational or linear. A carrier may be used to retain the wafer while it is pressed against the top surface of the polishing pad. A carousel apparatus or other means may be used to transport the carrier, and the wafer held by the carrier, over the polishing pad before polishing and away from the polishing pad after polishing of the wafer.